The present invention generally relates to a buzzer and, more particularly, to a piezoelectric buzzer employing a piezoelectric composite as a source of a buzzing sound.
Various types of piezoelectric buzzers are currently commercially available, one of which will now be described with reference to FIGS. 1 to 4 of the accompanying drawings.
The prior art piezoelectric buzzer comprises a casing 10, shown in a bottom plan view and in a side sectional view, respectively, in FIGS. 3 and 4, and a piezoelectric composite generally identified by 12 and housed within the casing 10 as best shown in FIG. 4. As best shown in FIGS. 3 and 4, the casing 10 comprises a cylindrical hollow body 14 having one end formed with a pair of spaced mounting lugs 16, opposed to each other and extending radially outwardly therefrom, and the other end closed by an end wall 18 having an opening 20 defined therein concentrically with said end wall 18. The casing 10 further comprises a spacer ring 22 of an outer diameter equal to the inner diameter of the cylindrical hollow body 14, which spacer ring 22 is positioned inside the hollow body 14 and connected to the outer peripheral portion of the end wall 18. In practice, the casing 10 is of one-piece construction with all of the component parts identified by 14, 16, 18 and 22.
The piezoelectric composite 12 comprises, as best shown in FIGS. 1 and 2 in a top plan view and a side sectional view, respectively, a vibrating disc 24 which has a diameter equal to the inner diameter of the hollow body 14 and which may be made of a thin metallic sheet, a layer 26 of piezoelectric material deposited on one surface of the vibrating disc 24, and an electrode layer 28 made of an electroconductive material such as silver or nickel and deposited on one surface of the piezoelectric layer 26 remote from the vibrating disc 24. This piezoelectric composite 12 is positioned within the hollow body 14 with a peripheral portion of the vibrating disc 24 bonded to the spacer ring 22 in spaced relation to the end wall 18 by the use of a bonding agent and with the piezoelectric layer 26 facing in a direction opposite to the end wall 18 as best shown in FIG. 4.
The prior art piezoelectric buzzer of the construction described above and shown in FIGS. 1 to 4 is so designed that, when an electric signal is applied between the electrode layer 28 and the vibrating disc 24, the layer 26 of piezoelectric material is brought into vibration in the axial direction of the vibrating disc 24. Therefore, the piezoelectric composite 12 comprising the layer 26 and the vibrating disc 24, both attached to each other to form a unitary body, vibrates in a bending mode of vibration thereof.
In the prior art piezoelectric buzzer, when the piezoelectric composite 12 vibrates in the manner described above, the initial node of vibration lies in a circular line, shown by the broken line n in FIG. 1, coaxial with the piezoelectric composite 12 and extending inside the vibrating disc 24 in substantial alignment with the periphery of the piezoelectric layer 26, with the maximum amplitude of vibrations taking place at the center of the composite 12 and also at the peripheral portion 30 of the vibrating disc 24. The vibration of the piezoelectric composite 12 within the casing 10 causes the casing 10 to resonate.
With the prior art piezoelectric buzzer of the type referred to above, since the mounting lugs 16 are integral parts of the casing 10 and are, therefore, rigid with the hollow body 14, the vibration of the piezoelectric composite 12 tends to be suppressed, when the piezoelectric buzzer is connected to a support surface, for example, a wall, with set screws threaded through the mounting lugs 16 into the supporting surface, thereby resulting in reduction of the volume of sound produced and also displacement of the frequency of vibration.